Step 1: Understanding the Concept: 
The field of individual differences in psychology studies the ways in which people differ from one another. The question asks for a comprehensive definition. 
Step 2: Detailed Explanation: 
(A) Difference in individual's characteristics: This refers to enduring traits and attributes such as personality (e.g., introversion vs. extroversion), intelligence, abilities, values, and physical traits. This is a core part of individual differences. 
(B) Difference in individual's behaviour pattern: This refers to the observable ways people act and react in various situations. These behavioral patterns are often the expression of the underlying characteristics mentioned in (A). 
(C) Both (A) and (B): Since individual differences encompass both the underlying psychological characteristics and the resulting observable patterns of behavior, a complete definition must include both aspects. Differences in characteristics lead to differences in behavior. 
Step 3: Final Answer: 
The meaning of individual difference includes variation in both internal characteristics and external behavior patterns. Therefore, "Both (A) and (B)" is the most accurate answer.